In bitcoin you can have a transaction which requires 2 or more people to sign it before the transaction can happen/is considered valid at all. Smart contracts,

In bitcoin you can have a transaction which requires 2 or more people to sign it before the transaction can happen/is considered valid at all. Smart contracts, is specifying conditions which can be checked in the blockchain, like existence of certain data you expect to be posted in the OP_RETURN (comment-like field) by other transactions. When your conditions are met the "smart-contract" makes/signs and publishes its own transaction, and other contracts can depend on this output of your contract.